Recently I started training myself speed reading skills, and began to wonder whether it's possible to have such a widget on my laptop.

The basic idea is actually quite simple: draw vertical lines on the screen and make them "always on top". The feature list is is also minimum: be able to shift those lines horizontally. Of course, more features could be added, like line color or thickness. With those lines guiding the eyes, user can speed-read the background pdf, webpage, ebook, etc. An example of this idea is attached.

I myself have no experience developing screenlet, AFAIK, it is written in javascript, along with some other files. As so many of screenlet/applets/... have been developed, I hope someone is willing to turn this idea into implementation -- you own all the credit of course.